1. Field of the Invention
This invention relates to a radiation image recording and read-out method and an apparatus for carrying out the method. More particularly, this invention relates to a radiation image recording and read-out method in which a stimulable phosphor is once exposed to a radiation to have a radiation image stored therein, the stimulable phosphor is scanned with stimulating rays which cause the stimulable phosphor carrying the radiation image to emit light in the pattern of the radiation image stored therein, the emitted light is photoelectrically read out to obtain an electric signal, and a visible image is reproduced by use of the obtained electric signal, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to a stimulating ray such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264, it has been proposed to use a stimulable phosphor in a radiation image recording and read-out method, particularly for medical diagnosis. Specifically, a recording material comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with a laser beam which has a wavelength within the stimulation wavelength range for the stimulable phosphor and which causes the stimulable phosphor to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT).
This radiation image recording and read-out method using the stimulable phosphor is advantageous over conventional radiography using a silver halide emulsion, for example, in that the electric image signal used for reproducing the visible image can be freely processed according to the structure of the object to improve the image quality for viewing, particularly for diagnostic purposes, and further in that the radiation dose to the object can be greatly reduced by adjusting the read-out gain of the photodetector used to convert the light emitted from the stimulable phosphor upon stimulation thereof to the electric image signal.
In the radiation image recording and read-out method described above, it is desired that the amount of light emitted from the recording material upon scanning thereof with a laser beam (i.e. the read-out amount) be as large as possible. This is because, when the read-out amount is large, conversion of the light emitted from the stimulable phosphor of the recording material into the electric image signal can be conducted at a high signal-to-noise ratio (S/N ratio), and therefore it is possible to reproduce a visible image of high quality. The read-out amount per unit area of the recording material is given by the product of stimulation energy of the stimulating rays (laser beam) applied to the recording material and the read-out efficiency, which is a variable depending on the wavelength of the stimulating rays. Accordingly, in order to improve the read-out amount, it is necessary to increase the stimulation energy and further to scan the recording material with stimulating rays having a wavelength providing a high read-out efficiency.
Aforesaid U.S. Pat. No. 4,258,264 discloses a radiation image recording and read-out method in which a radiation image stored in the recording material is read out by stimulating the recording material by use of a laser beam having a wavelength within the range of 600 nm to 700 nm and photoelectrically detecting light having a wavelength within the range of 300 nm to 500 nm among the wavelengths of light emitted from the recording material upon stimulation thereof. U.S. Pat. No. 4,258,264 also suggests that the reason for the use of the laser beam having a wavelength within the range of 600 nm to 700 nm as the stimulating rays is that, in general, a high read-out efficiency can be obtained within this wavelength range of stimulating rays. As examples of laser sources emitting light having a wavelength within the aforesaid range, U.S. Pat. No. 4,258,264 mentions an He-Ne laser (633 nm), a Kr.sup.+ laser (647 nm) and a Rhodamine B dye laser (610 nm to 680 nm).
When medical diagnosis is conducted by use of the above-mentioned radiation image recording and read-out method, it is often necessary to sequentially record radiation images of patients at short recording intervals, for example, as in the case of mass medical examinations. In such a case, the radiation image recording and read-out method is required to be able to quickly read out a radiation image from the recording material after the radiation image is recorded thereon. Thus, there is an increasing need for a radiation image recording and read-out method capable of quickly reading out a radiation image from the recording material. Further, for carrying out such a method, it is practically desired to use an apparatus in which the recording material repeatedly used for recording radiation images is incorporated into a single unit together with a recording section for recording radiation images of patients on the recording material and a read-out section for reading out the radiation images recorded on the recording material. Since the stimulation energy per unit area of the recording material is given by the product of the output of the stimulating rays employed for the scanning of the recording material and the read-out time for the recording material, when the stimulation energy is constant, the read-out time for the recording material can be shortened by increasing the output of the stimulating rays.
As described above, in the aforesaid radiation image recording and read-out method, it is desired that the read-out amount in the scanning of the recording material with stimulating rays be as large as possible. Further, the radiation image recording and read-out method is desired to be able to quickly read out a radiation image from the recording material after the radiation image is recorded on the recording material. As explained above, the read-out amount per unit area of the recording material is given by the formula: EQU read-out amount=read-out efficiency.times.stimulation energy
in which the read-out efficiency is a variable depending on the wavelength of stimulating rays. On the other hand, the stimulation energy per unit area of the recording material is given by the formula: EQU stimulation energy=read-out time.times.stimulating ray output.
Accordingly, EQU read-out amount=read-out efficiency.times.read-out time .times.stimulating ray output.
As indicated by this formula, in order to shorten the read-out time for the recording material and still increase the read-out amount, it is necessary to greatly increase the output of stimulating rays.
The inventors conducted various experiments to find a radiation image recording and read-out method employing a high-output laser beam as the stimulating rays so that the stimulation energy can be increased even when the read-out time for the recording material is shortened, making it possible to increase the read-out amount. Surprisingly, these experiments revealed that dependence of the read-out efficiency on the wavelength of the stimulating rays changes according to the stimulation energy. More specifically, it has been found that a higher read-out efficiency is obtained with stimulating rays having a wavelength within the range of 600 nm to 700 nm when the stimulation energy is low, but the read-out efficiency is improved with stimulating rays having a wavelength shorter than 600 nm over the read-out efficiency obtained with the stimulating rays having a wavelength within the range of 600 nm to 700 nm as the stimulation energy increases. Accordingly, when a laser beam having a considerably high output like a gas ion laser beam is used as the stimulating rays to shorten the read-out time for the recording material and still increase the read-out amount, a higher read-out amount can be obtained by selecting a laser beam having a wavelength shorter than 600 nm rather than a laser beam having a wavelength within the range of 600 nm to 700 nm.